In recent decades, an increasing trend used by the oil and gas industry for producing natural gas from unconventional resource plays has been the application of hydraulic fracturing, or fracking. Examples of unconventional plays include tight sand formations containing natural gas, and shale reservoirs, such as the Marcellus shale formation located in the Northeastern United States.
Fracking involves the introduction of a fracking fluid at high pressure into a formation to cause stress fractures in the surrounding formation. This increases the permeability of the formation, thereby allowing more of the natural gas trapped within the formation to be collected. After fracturing, a proppant is introduced to keep open the resultant fractures. Fracking has proven to be revolutionary to the natural gas industry, and proponents of fracking believe it to be a major step for reducing dependence on foreign sources of natural gas. However, opponents claim that fracking fluid may contaminate ground water and often propose greater restrictions and better regulated procedures than those currently employed. The oil and gas industry has responded to concerns by attempting to map out the induced fracture network in conjunction with the use of “friendlier” fracking fluids. By mapping the induced fracture network, information may be obtained regarding the extension of the induced fractures and the proximity of the induced fractures to other underground features, such as underground faults. If induced fractures reach a natural fault that extends toward the surface, frac fluid may be able to migrate upward, approaching more shallow underground features such as the aquifer.
In an attempt to map induced fractures in typical fracking systems, micro-seismic arrays are arranged on the surface or placed down hole, and configured to listen for “pops” which occur when induced shear fractures intersect with existing natural fractures. Release of energy associated with the pops or seismic events travels via elastic wave propagation to an array of geophones (e.g., receivers) which are used to triangulate the location of the event or hypocenter. The seismic data is collected and triangulation calculations are performed to locate the source of the seismic activity. However, seismic sensors suffer from reduced sensitivity due to very weak signals and the attenuation of seismic signals by natural fractures. Additionally, there is a high degree of uncertainty in the measurement of micro-seismic activity due to the inability to know a priori when a fracture event is going to occur. The seismic monitoring approach is also known as passive micro-seismic monitoring, in which the term passive refers to the lack of controlled energy input resulting from the use of explosives, air guns, or thumpers for reflection seismic surveys. Because there is no controlled energy input, the event time for a shear pop is unknown. Thus, there are more unknowns to solve for and the mapping problem is more difficult. Also, there may be overlapping pops, which complicate the process of sorting out the seismic return signals (e.g. first arrivals).
Another approach involves the placement of very sensitive tilt meters which act like a carpenter's level to measure movement of the earth's surface due to expansion and shifting of subsurface formations due to the introduction of a pressurized fluid and the resulting induced fracture network.
Still another approach has been suggested in which radioactive isotopes are added to the fracking fluid and subsequently monitored using gamma ray spectrometry. However, this approach introduces additional potentially dangerous substances into the ground, where the trend is toward less invasive materials. These known methods fail to relate any information regarding proppant injected into induced fractures during hydraulic fracturing. For example, rock may be fractured under pressure, but the fractures may close once the source of force against the fracture is removed or reduced. Therefore, even if the known methods perform as intended, a true picture of the induced fracture network is not obtained. Alternative methods and systems for mapping induced hydraulic fractures are desired.